Vertices moving at different rates?
Move your 3D Cursor around might fix the issue.

The amount of mouse movement converted into 3D translations are based on the distance between the local/selected pivot and the view pivot.  If you set the pivot point to 3D cursor, then the position of the 3D cursor will affect how much of your mouse movement is converted into actual 3D distance.

Objects' scaling transformation also has some play in this.